1. Field of the Invention
The present invention relates to a nonlinear equalizer and a decoding circuit and method using the same and, more particularly to a circuit and method having a nonlinear equalizer to eliminate intersymbol interference (ISI) in high density recording systems.
2. Description of the Related Art
Digital magnetic recording systems typically experience severe nonlinearity at high densities. As explained by W. Zeng and J. Moon in xe2x80x9cSystems Modeling of Nonlinear Effects in High Density Digital Magnetic Recordingxe2x80x9d (IEEE GLOBE.COM., Vol. 2, pp. 1139-1143, 1994), this nonlinearity is caused by the strong interactions between the adjacent written transitions. The demagnetization field of a written transition can cause the shift in position and broadening of the next transition. Adjacent transitions also tend to erase each other, causing significant reduction in the signal amplitude. Such nonlinear distortion is commonly referred to collectively as nonlinear intersymbol interference (ISI).
Decision Feedback Equalization (DFE) is a well-known scheme used to detect signals transmitted across communications and recording channels with intersymbol interference. FIG. 1 shows a block diagram of a decoding circuit utilizing DFE 100. Such a circuit is typically used to recover data from a magnetic media or a communication channel. The analog signal from the read head is filtered by analog filter 102. The filtered analog signal is then applied to digital detection channel 104. Digital detection channel 104 includes an analog to digital converter (ADC) 106, a forward equalizer 108, threshold detector 110, decoder 112 and feedback equalizer 114. The ADC 106 samples the analog read signal at timing intervals defined by sample clock signal 116. The sampled read signal for digital detection channel 104 is equalized by forward equalizer 108. The binary data is detected by the threshold detector, 110 and input to decoder 112, which generates the final output data from the read circuit.
In the data recovery circuit utilizing DFE 100, the forward equalizer 108 shapes the readback pulse in a desired way and the feedback equalizer 114 attempts to cancel the nonlinear ISI. However, the feedback equalizer 114 cannot sufficiently eliminate nonlinear ISI in high density recording systems.
Another conventional solution to the problem of nonlinear ISI is a modified version of a DFE called a RAM-DFE. As explained by W. G. Jeon, J. S. Son, and Y. S. Cho, in xe2x80x9cNonlinear Equalization for Reduction of Nonlinear Distortion in High-Density Recording Channelsxe2x80x9d (IEEE International Conference on Communications, Vol. 1, pp. 503-507, 1995), in RAM-DFE, the feedback section of the decision feedback equalizer is replaced by a look-up table filter, so that this feedback table removes trailing nonlinear ISI that exists in the feedforward filter output. The feedforward section of the RAM-DFE remains linear.
However, because the feedforward section of the RAM-DFE has the same structure as the conventional DFE, it is not effective in reducing the nonlinear ISI existing in the precursor part. In addition, it requires a large memory of 2N (where N is the number of consecutive inputs) and the convergence speed is slow because of the search time needed for the look-up table filter.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a nonlinear equalizer which substantially removes intersymbol interference from a signal, particularly in a magnetic recording system, and which is faster and requires less memory than conventional decoding circuits.
In a first aspect, a nonlinear equalizer employs a signal conditioning algorithm for conditioning a partial response sampled signal having four nonzero samples into a three level signal. The nonlinear equalizer substantially eliminates intersymbol interference in an encoded analog signal. Further, the partial response sampled signal having four nonzero samples may include a PR4W sampled signal.
The nonlinear equalizer""s signal conditioning algorithm may condition the partial response sampled signal having four nonzero samples to produce a plurality of samples S1, S2, S3, and if sample S2 is greater than 0.4 in absolute value, the magnitudes of samples S1, and S3 may be decreased by a predetermined rate by adding xe2x88x92axc3x97(the sign of S2). The predetermined rate may be set, for example, at 0.2.
In another aspect, a decoding circuit includes an analog-to-digital converter for sampling an analog signal and outputting a partial response sampled signal having four nonzero samples, a linear equalizer for adjusting an amplitude and phase relations of the partial response sampled signal having four nonzero samples, a nonlinear equalizer for conditioning the partial response sampled signal having four nonzero samples and outputting a partial response sampled signal having two nonzero samples, and a partial response maximum likelihood detector, for detecting the partial response sampled signal having two nonzero samples. The decoding circuit may be used in a high density recording system. The decoding circuit may further include an analog filter for filtering the analog signal and outputting a filtered analog signal to the analog-to-digital converter, and a decoder, for inputting the partial response sampled signal having two nonzero samples from the partial response maximum likelihood detector and outputting a decoded data signal.
The nonlinear equalizer may modify samples independently of binary decisions. In addition, if an amplitude of a middle sample meets a predetermined condition, the nonlinear equalizer may modify previous and past samples by a predetermined amount.
In another aspect of the present invention, a method for recovering information coded onto an analog signal and recorded onto a tracked storage medium includes sampling the analog signal to produce a partial response sampled signal having a sequence of four non-zero samples S1, S2, S3, S4, conditioning the partial response sampled signal according to a signal conditioning algorithm to produce a three level signal, detecting the three level signal using a partial response maximum likelihood detector, and decoding the three level signal to produce a decoded data signal.
In another aspect, a magnetic recording system includes the above-mentioned decoding circuit. As noted above, the partial response sampled signal having two nonzero samples may be substantially devoid of intersymbol interference.
The inventive decoding circuit may also be part of an information handling/computer system. In addition, the present invention includes a programmable storage medium (e.g., a diskette) tangibly embodying a program of machine-readable instructions executable by a digital processing apparatus for performing the inventive method.
The unique and novel aspects of the present invention combine to provide an inventive decoding circuit which substantially removes intersymbol interference from a coded analog signal in a magnetic recording system and is faster and requires less memory than conventional decoding circuits.